Magnesium alloys, which have a high specific strength, are the lightest of alloys, are applicable in a variety of casting and wrought processes, and have a wide range of application, and are thereby used in almost all fields in which light weight is required, such as parts for vehicles and electromagnetic parts. However, magnesium (Mg) is a metallic element that has a low electrochemical potential and is very active. Mg still has limitations in terms of the stability and reliability of the material, since it undergoes a strong reaction when it comes into contact with oxygen or water, and a commercial Mg alloy mostly has an ignition temperature of below 550° C., which sometimes causes fires. Therefore, the application fields in which Mg can be applied are still limited compared to its potential applicability. In particular, it cannot be used in applications in which safety is important.
Further, research carried out into a magnesium alloy to date has only concentrated on a casting alloy which is adaptable to an engine, gear parts, or the like of a vehicle based on excellent castability of Mg, but, at present, there is a shortfall in research on a wrought magnesium alloy in the form of extrusion or plate which, due to its excellent mechanical properties, can be more diversely applied to the fields in which weight reduction is required.
As shown in FIG. 1, a precipitation-hardened Mg—Sn alloy has a high melting point in an eutectic structure and excellent thermal stability, and thus excellent hot-workability, compared to a commercial Mg—Al alloy. As shown in FIG. 2, it can be seen that the Mg—Al alloy shows a tendency to considerably decrease in extrusion rate when Al content increases for high strength, whereas the Mg—Sn alloy has a very high extrusion rate of 20 m/min or more even when 10% by weight of Sn is added. Further, as disclosed in Korean Patent No. 10-0994812, an Mg—Sn alloy is added with zinc (Zn) and aluminum (Al), and a resulting mixture is then extruded and heat-treated to enhance structure refining and precipitation hardening and solid-solution hardening effects, thereby forming an extruded Mg alloy having high strength and ductility. Particularly, in manufacture of the above alloy, it is essential that a billet cast prior to extrusion be treated with a homogenization annealing process at 480 to 520° C. for 0.5 to 24 hours.
However, since the Mg—Sn alloy has an ignition temperature of 400° C. or less and thus poor ignition resistance, it is required that a vacuum or shielding gas such as SF6 be used in performing the homogenization annealing process. However, there are problems in meeting the conditions in that addition of a vacuum apparatus to create a vacuum increases manufacturing cost, or SF6 is expensive and is classified as a greenhouse gas, the global-warming potential (GWP) of which is 23,900 times that of CO2, so that the use thereof is expected to be regulated in the future time. A further problem is that, in the case of performing heat treatment using a conventional heat-treating furnace that is commercially available, even when shielding gases are supplied to the inner wall of the furnace, a fire risk is still high there because the shielding effect with respect to the outside is not perfect.
Therefore, in order to basically suppress the fire risk during heat treatment and to maximize mechanical properties of an Mg—Sn alloy, it is necessary to develop an alloy in which ignition resistance thereof is improved without degradation of entire mechanical properties, thereby being capable of being heat treated at a temperature of 480° C. or more in the air or under a general inert atmosphere.